Just before 8pm last Wednesday Erika La Nardo, 16, hammered on a neighbour's door with the news which would shock Italy. Two men, she stammered. Two men had broken into her house. They had a knife. Her mother, her brother, bleeding, dead maybe. Oh God please help.

Police arrived minutes later, bursting into a crime scene they will not easily forget. Susy was slumped on the kitchen floor in a widening pool of blood. A slim, 45-year-old housewife, she was unrecognisable, repeatedly slashed and stabbed.

Her son, Gianluca, 12, was in a bath upstairs. If anything his wounds were worse, the victim of berserks who did not know when to stop. Mother and son were stabbed a total of 97 times.

The suburban townhouse in Novi Ligure, a small, prosperous town in the north-eastern region of Piedmont, was a slaughterhouse, gasped one detective. A wooden-handled, 15cm kitchen knife was recovered. Of the killers there was no sign.

Francesco La Nardo, Susy's husband, had been playing football during the butchery. A manager at a local chocolate factory, he had no known enemies.

All police had to go on was Erika's description of the intruders: one in his 40s, one in his 20s. They did not speak but had dark hair and sallow skin, typical of Slavs, of illegal immigrants.

Word spread and within hours a furious crowd surrounded the mayor. For months, years, there had been warnings about roving Albanian thugs. Yet the mayor did nothing. The police did nothing. And now Susy and Ginaluca were dead.

As Erika was shown police photos of Albanian criminals the cry went up around the country. Newspapers demanded a crackdown on illegal immigrants.

The Northern League, part of the centre-right opposition coalition, announced a series of demonstrations. Liberals squirmed and conceded that, yes, something would have to be done.

Meanwhile, investigators made some discoveries. Erika said she ran to her neighbour but bloodstained footprints showed she walked. Erika identified the photo of an Albanian who could not possibly have been in Novi Ligure that night. Erika said the intruders were thieves but nothing was missing.

Erika was lying. In a series of twists that have stunned Italy, Erika and her 17-year-old boyfriend have been arrested for the murders. A secret surveillance camera and microphone inside a police interview room caught the couple discussing the killings.

In an echo of the Stephen Lawrence-murder suspects, Erika was filmed miming the stabbings. They are expected to be charged this week. The case has dominated headlines and led news bulletins for seven days.

Italy is peering into its soul and finding only darkness. It is telling that Erika chose to blame immigrants. Linking them to crime is a national pastime: the number of Italians making the connection is twice that of Britons, according to a recent poll.

Politicians, notably those from the centre-right, led by Silvio Berlusconi, say it is a fact. Cracking down on immigrants and crime are electric campaign issues in elections due in May. The ruling centre-left has appealed for tolerance but cannot resist promising its own populist crackdown.

Justice minister Piero Fassino summed up the newfound sense of unease: "I believe that someone should beg the pardon of immigrants. The level of incivility we've seen in this country in the last 48 hours should make us all reflect."

The mayor of Novi Ligure went further: "We've gone from one horror to another." Such sentiments are being overtaken by bewilderment that apparently normal, happy teenagers could be responsible. Motive remains a mystery

Last night Erika and her boyfriend, named only as Mauro, were still accusing each other, giving completely different versions of who instigated the violence.

They agree only that Erika gave Mauro some fruit juice from the fridge before Susy returned home with Gianluca. The horror that followed is tormenting Italy.

Is there a malady at the root of family life, an emptiness in adolescents, a fraying moral fabric? Psychologists, sociologists and psychiatrists have been called on to explain. None has succeeded. "We have met the enemy and discovered that it is ourselves," said Corriere della Sera.